1. Field of the Invention
The present invention relates to an image forming apparatus using an electrophotographic process.
2. Description of the Related Art
Today, an image forming apparatus, such as a copying machine and a printer, using an electrophotographic process, is demanded to perform image formation at an even higher speed, and hence, as a means for forming an electrostatic latent image on an image bearing member, there is employed a laser optical system that emits a laser beam from a semiconductor laser to a polygonal mirror (rotary polygonal mirror). The polygonal mirror is a polyhedron having four to twelve faces, and is irradiated with the laser beam while being rotated at high speed, whereby formation of an electrostatic latent image in a main scanning direction (i.e. in the longitudinal direction of the image bearing member) is realized.
A motor for driving the polygonal mirror for rotation is called a scanner motor. The scanner motor starts rotation when a user presses a copy start key or the like to cause the apparatus to start a copying operation. A stable latent image cannot be formed before the scanner motor comes to rotate stably at a predetermined speed, and therefore the image forming apparatus is controlled such that a shift to an image forming sequence takes place only after a sign indicative of stabile rotation of the scanner motor has been detected.
Since the scanner motor rotates the polygonal mirror, it has a large inertia, so that it is known to take long time before the rotation of the scanner motor becomes stabile. Further, it is necessary to cause the scanner motor to rotate at a very high speed in accordance with a recent increase in the operating speed of the image forming apparatus, and hence it tends to take more time (spin-up time) from when the scanner motor at rest is started to when the scanner motor comes to stably rotate at a predetermined speed.
An image forming operation cannot be started unless the scanner motor is in a stable rotating state, and hence, in general, in the image forming apparatus using the laser optical system, a first copy (print) time, i.e. a time period from pressing of the copy start key to output of a first sheet depends on a time period required for the scanner motor to spin up.
In view of this, an image forming apparatus has been proposed in Japanese Laid-Open Patent Publication (Kokai) No. H07-273951, which has a function of starting rotation of the scanner motor prior to pressing the copy start key so as to shorten the first copy time and reduce the load on the user.
However, the prior rotation control of the scanner motor is executed before the copying operation is started (before a copy start instruction signal is generated), based on a prediction that a user will start a print (copy) job (i.e. in response to a sign of image formation, i.e. in response to an operation on the image forming apparatus), but the print job is not necessarily started after execution of the prior rotation control.
When the print job is not carried out after all, the prior rotation control of the scanner motor comes to naught. More specifically, although the image formation is not carried out, the scanner motor is started, caused to rotate over a predetermined time period, and then stopped. This is not preferable to the service life of the scanner motor.
Further, it is known that since the scanner motor steadily rotates at a very high speed, the service life of the scanner motor is also limited in terms of maintaining stable rotation. After the service life of the scanner motor expires, it becomes impossible to maintain stable rotation or it takes a longer time period for the scanner motor to be started from the stopped state to reach the stably rotating state.
Since the polygonal mirror driven by the scanner motor reflects a laser beam for forming a latent image on the image bearing member, if the scanner motor cannot steadily rotate with stability, the latent image on the image bearing member is distorted, which causes degradation of image quality.
As described above, in the case where the image forming apparatus is designed such that the prior rotation of the scanner motor is not executed, it is impossible to shorten the first copy time, while in the case where the prior rotation of the scanner motor is executed, reduction of the service life of the scanner motor, in particular, is accelerated. For these reasons, it could not help but design the laser optical system such that highly durable expensive motor components are used therein beforehand.